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Epidemiology

This appearance may be apparent in approximately 4% of those with pulmonary sarcoidosis on plain film 1 and up to 15% on CT 2.

Pathology

This appearance is thought to result from the aggregation of a vast number of interstitial granulomas rather than representing a true alveolar process. Some authors have therefore applied a more appropriate term, pseudoalveolar sarcoidosis.

Radiographic features

CT/HRCT chest

There can be large areas of pulmonary opacification ranging in diameter from 1 to 4 cm. These can be rounded or elongated in shape, have irregular edges and blurred margins, with or without air bronchograms. They are typically found either along the bronchovascular bundles or in the lung periphery adjacent to the pleural surface (i.e., subpleural) 3.

When these opacities are accompanied by multiple surrounding nodules, representing granulomas as well, the entire conglomerate is often termed the galaxy sign.

Another pattern of alveolar sarcoidosis is an appearance termed "fairy ring", which refers to circumferentially organised opacities. In this situation, a reverse halo sign may be seen.